Impact hammers come in a variety of sizes with a variety of hammer tips, with each hammer having its own strengths and weaknesses. A massive hammer with a softer tip is ideal for exciting the lower frequencies (putting in enough energy to yield enough SNR on microphones), while a lighter hammer with a nylon or metal tip is better for exciting the higher frequencies. The massive hammer cannot put energy into the higher frequencies, while the lighter hammer may introduce errors related to sensor base strain and overloads at the lower frequencies.
Within DIRAC, multiple hammers can be used within a single project to excite their respective frequency ranges. This allows to get accurate results over a wider frequency range than using a single impact hammer alone. Within the measure module, multiple hammers can be defined along with the desired cut-off and roll-off frequencies for the filtering operation. The operations reflect well-known analog filters such as low-pass, high-pass and band-pass filters, however the implementation uses multiplication of the input/output spectra in the frequency domain. This has the benefit that there is no distortion of phase, and that the parts of the spectrum which are “filtered out” are really set to zero. Further details are found in 5.2 Measurement wizard.
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